1. Field
This invention relates to bifocal contact lenses and bifocal intraocular lenses with upper and lower optical power zones.
2. Prior Art
Bifocal contact lenses are lenses with at least two regions of different optical powers, known as zones or segments. Usually, one power is chosen to provide the wearer with clear distance vision and the second power to provide clear near vision, but any two powers may be selected. Bifocal contact lenses also may be called multifocal contact lenses, although the latter term is sometimes reserved for lenses comprised of at least three regions with different optical powers or regions of variable power, as in U.S. Pat. Nos. 5,517,260 (Glady) and 5,754,270 (Rehse.) Bifocal contact lenses have some features in common with bifocal intraocular lenses and some differences.
Bifocal contact lenses generally are classified into two types, concentric and vertically segmented. Both types can be produced as rigid or soft contact lenses.
Concentric bifocal contact lenses have a central power zone surrounded by one or more annular zones of different powers or a sequence of alternating powers. Generally, the lens is designed so as to have little motion on the eye and the wearer views through portions of more than one zone at the same time, a process called simultaneous vision, as described in U.S. Pat. Nos. 4,636,049 (Blacker); 4,752,123 (Blacker); 4,869,587 (Breger); and 5,864,379 (Dunn). The distance and near zones, together with optional transition curves, comprise the bifocal area. The peripheral portion of the lens is comprised of one or more curves that are used to connect the bifocal area to the edge perimeter, including options currently in use such as prism ballast, slab-off, tapers, peripheral curves, lenticular curves, truncations and.
Vertically segmented bifocal contact lenses have vertically separated power zones, an upper zone that usually provides the appropriate correction for viewing far distances and a lower zone, which usually provides the appropriate correction for viewing near distances. The lenses are designed to alternate their position in front of the pupil when the lens moves up and down on the eye as the result of lid forces, which occur when the wearer changes gaze between different distances, a process called alternating vision, as described in U.S. Pat. Nos. 3,597,055 (Neefe) and 3,684,357 (Tsuetaki). If there is little vertical movement then vertically segmented bifocal contact lenses may also function as a simultaneous vision lens.
The two vertically separated power zones maintain their relative positions by various features that can be added to control the lens position and stabilize the meridional rotation as described in U.S. Pat. Nos. 4,095,878 (Fanfi); 4,268,133 (Fischer); 5,760,870 (Payor); 5,296,880 (Webb); and 4,573,775 (Bayshore). This is commonly accomplished in rigid bifocal contact lenses by incorporating a prism into the lens, which provides a progressively greater thickness from the top to the bottom of the lens. The prism serves to maintain the desired lens orientation and keep the lower zone of the lens downward on the eye as described in U.S. Pat. Nos. 5,430,504 (Muckenhirn) and 4,854,089 (morales) and in Burris, 1993; Bierly, 1995, and Conklin Jr. et al, 1992. The lower edge of the lens is designed to rest upon the lower lid margin of the wearer and the lens shifts up and down relative to the eye as the result of lid forces. There are several subtypes of vertically segmented bifocal contact lenses, based on the shape of the near zone, including round, D-shaped, flat, crescent, and others as described by Conklin Jr. et al, 1992 and in U.S. Pat. No. 4,618,229 (Jacobstein) and U.S. Pat. No. 5,074,082 (Cappelli).
There have been attempts to incorporate prism into soft bifocal contact lenses for the same functional purpose as prism provides for rigid lenses. U.S. Pat. Nos. 4,549,794 (Loshaek); 5,635,998 (Baugh); 4,618,229 (Jacobstein) Ezekiel, 2002, but generally these lenses have inadequate lens movement or produce discomfort to the wearer. There also have been attempts to induce a vertical shift of a soft bifocal contact lens by adding features to the lower periphery of the lens, as described in U.S. Pats. 4,614,413 (Obssuth); 5,635,998 (Baugh); 6,109,749 (Bernstein): 5,912,719; and European Pat. EP0042023 (Muller).
A more successful soft bifocal contact lens design (U.S. patent application Ser. No. 09/908,296 (Mandell)) contains at least two prisms. A primary prism controls lens positioning and meridional orientation, while a secondary prism controls lens movement.
Transition
The power zones of a bifocal contact lens contain different surface curvatures, which are linked together by a transition. The transition may have zero or finite width and may vary in design, depending upon how the adjacent power zones are oriented with respect to each other.
If a bifocal contact lens has a transition of zero width and there is a change in slope of the adjacent zones at their junction, the transition will appear as a line when viewing the front surface of the lens as shown in U.S. Pat. No. 4,752,123 (Blacker). If there is no change in slope, the transition will appear smooth and will not be detectable by surface inspection.
Concentric bifocal contact lenses are available with a greater variety of transition designs than are found in vertically segmented lenses. An example is shown in FIG. 1, which illustrates the midline cross-section construction of a prior-art concentric bifocal contact lens 14. On a front surface 15 a distance power zone 16 is located in the center of the lens and a near power zone 17 surrounds the distance zone in a concentric arrangement. A center of curvature for the distance zone 18 and another for near zone 19 lie on a common axis of symmetry, so that at a transition 20 between the zones there is not only a change in curvature, but also a change in slope that is equal to an angle 21 between a radius for the distance power zone 22 and a radius for the near power zone 23. The front surface 15 of lens 14 has a visible transition line in the shape of an arc of a circle. Distance zone 16 and near zone 17 comprise a bifocal area 24, limited by a bifocal perimeter 25, which is surrounded by a peripheral zone 26 that extends to an edge perimeter 27.
FIG. 2 shows another example of a prior-art concentric bifocal contact lens front surface, which has center of curvature 19 for near zone 17 that occurs on lines connecting center of curvature 18 for distance zone 16 and transition 20. There is no slope change at transition 20 and there is no visible transition line when viewing the front of the lens. However, in three dimensions the center of curvature for the near segment is a locus of points that form a circle and the near zone is part of a torus, rather than a sphere. If the radius of the torus increases towards the periphery, near zone 17 is an aspheric curve. This arrangement can be used to connect a spherical distance zone to an aspherical near zone with no slope change at the transition. In a similar manner various combinations of spherical and aspherical curves can be combined to produce a variety of concentric bifocal designs having no slope change at the transition. A front view of the surface any of these of lenses does not show a visible transition.
There are several design options currently available for the transition of bifocal contact lenses with vertically segmented zones. One design is a front surface bifocal in which the center of curvature of the near zone is displaced upward with respect to the distance zone. This creates a transition in which the two adjacent zones join together at the same height, relative to the back surface, but with an instantaneous change in slope as revealed in U.S. Pat. No. 4,854,089 ((Morales). A front view of such a lens appears as a line that is arc shaped, concave upward.
Unfortunately, an abrupt change in slope at the transition between two adjacent power zones is accompanied by a prismatic optical difference, which causes the wearer of the contact lens to observe a change in the image position when his gaze is shifted between the distance and near zones, a phenomenon known as image jump. Most contact lens wearers find that image jump is disturbing and generally poorly tolerated. Furthermore, if the lens assumes a position on the eye such that the transition lies in front of the pupil, the prismatic difference of light passing into the eye simultaneously from both the distance and near zones will cause the wearer to experience image doubling, which is intolerable to the lens wearer.
In another prior-art bifocal contact lens design, image jump and doubling were avoided by making the bifocal contact lens monocentric.(R Mandell, 1967,1974, 1988) A monocentric bifocal lens is a lens that has no prismatic difference between the power zones at the transition (FIG. 3). Monocentricity can be produced on front surface 15 of bifocal contact lens 14 by locating center of curvature 18 for distance zone 16 and center of curvature 19 for near zone 17 on a common line that also passes through transition 20. Unfortunately, the optical advantage of monocentricity is accompanied by a physical limitation.
FIG. 4 shows how the two power zones of FIG. 3 can be made to coincide at a midpoint 28 of transition 20 but they do not coincide at peripheral points 29 on transition 20 due to the difference in curvature of the zones. The height of distance zone 16 along transition 20 increases towards the periphery relative to the height of near zone 17. Consequently there is a step up in height in passing from the near to the distance zone and the step increases towards the lens periphery. When the lens is worm, the transition step interrupts the smooth flow of the lid across the lens during blinking.
FIG. 5 shows the magnitude of the step height for a range of moncentric bifocal contact lens parameters. The step height increases with the power of the bifocal add as well as from the midpoint to the periphery of the transition. For example, given an add power of 3.00 diopters and an optic zone diameter of 8 mm the step height would range from zero at the midpoint to 0.052 mm in the periphery. Typical maximum values for step heights of bifocal adds between 1.00 and 4.00 diopters would range from about 0.02 to 0.07 mm.
The step height of the transition can be changed by a modification of the monocentric bifocal design. For example, the centers of curvature for the distance and near zones can fall on a common line with the transition, but the distance zone is displaced inwardly at the midpoint of the transition. A step down occurs from the near to the distance zone at the midpoint of the transition. The two zones have the same height at two points along the transition in the periphery, as in U.S. Pat. No. 4,549,794 (Loshaek).
Another option for the design of a moncentric bifocal contact lens is intermediate to the other two designs. The distance and near zones are positioned such that there is a step inward near the midpoint of the transition that is less than the step in Option 2. The step decreases towards the periphery until at some intermediate position on the transition there is no step, followed at more peripheral locations by a step outward that would increase towards the periphery.
In theory the step that occurs at the transition of the monocentric bifocal of FIG. 4 consists of an abrupt increase in height, which can be represented by a square wave function. The surface of the near zone reaches the transition and then an instantaneous height increase occurs in passing to the distance zone as shown in U.S. Pat. No. 5,245,366 (Svochak). In practice, a square wave function is not produced on the lens because of the constraints of manufacturing. For example, in using a standard lathe to manufacture the monocentric bifocal contact of FIG. 4, the entire surface of the lens is first generated using the radius of the distance zone. Then, the cutting tool is adjusted for the near radius and the center of rotation is offset to fall on the line connecting the centers of curvature for the distance zone and the transition point. Next the near zone is generated up to the transition, at which position the curvature of the cutting tool will be imparted into the portion of the near zone that is adjacent to the distance zone. The impression made by the cutting tool becomes the transition curve of the lens. The mathematical function that represents the transition shape is determined by the shape of the tool. Since the cutting edge of the tool is a convex circle or asphere, the transition that is formed will be a concave negative replica of the cutting tool.
Generally it is found that most cutting tools used in the manufacture of contact lenses have a radius of curvature at the cutting edge that is between 0.1 and 0.6 mm. Therefore, it is this same radius that will be found in negative form on the transition of the lens that is manufactured. The result is fortuitous because the curvature of the transition is more gradual than would be the case were an actual square wave function created. The more gradual slope of the transition creates a smoother surface for the lid to pass over and adds to the comfort of the lens when worn. However, a transition created by the shape of the cutting tool is not ideal since it presents abrupt slope changes in passing from the transition to the power zones. In addition, the width of the transition, and hence the rate of change of its slope, is limited by the radius of the lathe cutting tool.
An alternate method of manufacturing a monocentric bifocal contact lens can be achieved by using an oscillating lathe, such as the Precitech Optomform 40 with Variform Generator by Sterling Co. of Tampa, Fla. or DAC Series IV/2 Axis ALM by DAC International of Carpinteria, Calif. This type of lathe can be used to generate two or more zones on the front surface of a monocentric bifocal lens in a single continuous motion by varying the distance of the cutting tool relative to the lens back surface during each rotation of the lathe spindle. The cutting tool moves in and out from the lens surface with each cycle at the same time it traverses from lens edge to center. A problem is presented when the lathe tool passes between the distance and near zones, which are at different heights. The cutting tool cannot make an instantaneous change in height, which would require a square wave motion. Instead, the most efficient transition curve is used that will allow the lathe to change the height of the cutting tool as rapidly as possible. The curve that is usually chosen to do this is based on a sinusoidal function. This results in a transition curve that is smooth but which has a change in slope at the connections to the adjacent zones. In addition, the curve may not be smooth in a radial direction.
Another attempt was made at smoothing the transition between the distance and near zones of a bifocal contact lens is by using a mechanical device that changes radius of curvature while cutting the transition area, as in U.S. Pat. No. 5,430,504 (Muckenhirn). This device produces a curve for the transition that varies continuously from the radius of the distance zone to the radius of the near zone. This results in a curve instead of a step at the transition, but also produces a line at the boundary between the junction and the distance power zone, where a slope change is evident.
Intraocular (implanted) bifocal lenses are manufactured by essentially the same process as bifocal contact lenses, except for their biconvex shape. In these lenses, if there is a rough or abrupt transition it can cause the accumulation of debris in the, eye and produce adverse reactions. Although an intraocular bifocal lens might have restricted movement the optical advantage of monocentricity would still provide optimal simultaneous vision. The principles of the present invention for bifocal contact lenses would also apply to intraocular lenses.